Series pass disconnect is the two back to back power MOSFET's in many BMS's to disconnect the battery from inverter/charger. It is bi-directional which is why there is two back to back MOSFET to handle charge current and discharge current directions. The lower their series resistance the better to avoid BMS heating under higher current loads. Their only purpose is for safety to open the connection path to the battery. Normally they are always 'ON'. If they create too much heat and fail shorted they can create more of a safety issue then they are there to protecting against.
Using a starter solenoid relay to replace these is just dumb. Starter solenoid relays are intended for short momentary operation to run an engine starter, not for continuous operation. Sorry for being blunt. Only reasonable substitute relay is a latching high current relay which is what is used for a standby generator transfer switch but a generator transfer switch is very expensive and not rated for DC operation. DC operation relays, circuit breakers, or switches are more expensive because switching high current DC is more likely to 'weld' contacts together.
All the b.s. was to lead up to the last sentence.
I appreciate the perspective and explanation, thanks for sharing it and for clarifying. I think you make some good points, that probably deserve more attention/discussion on this forum. The robustness of the cheap commodity BMS' sold on Alibaba and elsewhere is often questionable at best, and the current ratings seem often unrealistic. On the other hand it many people use them (apparently) without issue.
I'm not sure that it follows that a high current
latching relay is the only proper solution. But I think that you are right that the protection system needs to be properly and safely designed to handle the continuous currents they will encounter (with some safety margin). And be rated for DC and the proper voltage range for the application. Whether or not FET based BMS are up to the task and/or where specifically the limit should be is over my head, but I certainly have more trust in more robust topologies.
One of the problems is there is no single definition of what a BMS is. In one of Will's video he implied an active balancer is not a real BMS. To a Tesla, with essentially individual charger control for every battery cell voltage level, all the Alibaba BMS's are not a real BMS.
I agree 'BMS' is a somewhat nebulous term.
I also agree with Will here. I don't think an active balancer meets the definition or purpose of a Battery
Management System.
There is naturally some vagueness and subjectivity as to where to draw the line on what is and isn't a BMS, but I think an active balancer squarely falls on the 'clearly not a BMS' side of the line. An active balancer performs one function cell balancing. It could be a component of a battery management system, but not one on its own. It doesn't manage the battery or charge/discharge, and can't act to protect the battery or the system.
I don't think the Tesla analogy works in this context either. An EV BMS, a cheap aliexpress BMS,and a smartphone BMS are certainly very different in terms of scope, capability, and sophistication. But they all perform the same broad function-- battery management and protection. As far as I understand (having never personally used or owned one), an active balancer isn't designed for or capable of performing this function. They are a single purpose item (as reflected by their name).
A BMS in my opinion is not defined by its sophistication, but by its function. A BMS for a cheap device with a small lithium battery pack needs to be small and low cost, a BMS for an expensive high power EV obviously needs (and can justify the cost of) a much more sophisticated, robust, and expensive BMS. Things like E-bikes, small to medium off-grid solar battery banks, etc, will fall somewhere in between. In other words the battery management system will naturally be proportional to (1) the cost of the system (2) the size/power/importance of the system.
This is my 2c at least, and that might be a little overpriced
